1. Field of the Invention
This invention relates to a developing roller for use in electrophotographic and electrostatic recording machines such as copiers and printers for supplying a one-component developer to an image forming body for forming a visible image on the surface of the image forming body. More particularly, it relates to a developing roller which is durable so that acceptable images can be reproduced without a drop of image density over a long period of time. It also relates to a developing apparatus using the same developing roller.
2. Prior Art
In conjunction with prior art electrophotographic and electrostatic recording machines such as copiers and printers, one typical developing method is an impression developing method as disclosed in U.S. Pat. Nos. 3,152,012 and 3,731,146. This developing method visualizes electrostatic latent images by supplying a one-component developer to a photoconductor drum bearing a latent image, thereby adhering the developer to the latent image. Since the impression developing method eliminates a need for magnetic material, the apparatus can be simple and compact and color toner can be used.
More particularly, the impression developing method is printed by bringing a developing roller carrying a toner, typically non-magnetic one-component developer in contact with a latent image holder or photoconductor drum bearing an electrostatic latent image, thereby adhering the toner to the latent image. The developing roller must be formed of an elastic material having electrical conductivity.
Referring to FIG. 2, the impression developing method is briefly described. A developing roller 1 is placed between a toner feed roller 4 for feeding a toner 6 and a photoconductor drum 5 having an electrostatic latent image borne thereon. Upon rotation of the developing roller 1, photoconductor drum 5, and toner feed roller 4 in the directions shown by arrows, the toner 6 is fed from the feed roller 5 onto the surface of the developing roller 1 and regulated into a uniform thin layer by a doctor blade 7. The thin layer of toner is then delivered from the developing roller 1 to the photoconductor drum 5 to adhere to the latent image whereby the latent image is developed into a visible toner image. The toner image is finally transferred from the photoconductor drum 5 to a recording medium, typically paper in a transfer section 8. Also included in the device is a cleaning section 9 having a cleaning blade 10 for scraping off the toner left on the photoconductor drum 5 after the transfer step.
During rotation, the developing roller 1 must maintain close contact with the photoconductor drum 5. The conventional developing roller 1 is of a structure having a conductive elastic layer 3 around a shaft 2 as shown in FIG. 1. The shaft 2 is of a highly conductive material, typically metal. The conductive elastic layer 3 is formed of a conductive elastomer in the form of an elastic rubber such as silicone rubber, acrylonitrile-butadiene rubber (NBR), and ethylene-propylene-diene terpolymer (EPDM) or a sponge such as urethane foam, with a suitable conductive agent being blended therein.
However, prior art developing rollers had the following problems resulting from the properties of elastic layers used therein. (1) Where the elastic layer of the developing roller is formed of an elastic rubber such as silicone rubber, NBR and EPDM having a low hardness enough to achieve tight contact, oil components contained in the rubber bleed out to the roller surface, which becomes sticky to the toner. This can cause a drop of image density after a long period of operation. (2) Where a spongy body such as urethane foam is used to form the elastic layer, toner can penetrate into pores in the elastic layer. As the penetrating toner accumulates from long-term use, the roller becomes harder and electrical charging of toner becomes inefficient, resulting in a drop of image density.
The latent image holder can be a belt instead of the drum. Another image forming system is known wherein toner is directly delivered from a developing roller to a recording medium in the form of a sheet of plain paper, OHP film and photographic paper. In these systems, the same developing roller can be used and the same problems will occur as in the aforementioned system.
Therefore, an object of the present invention is to provide a developing roller which ensures that images of quality are reproduced without a drop of image density over a long period of time. Another object of the present invention is to provide a developing apparatus using such a developing roller.